16
[0-0]:
[5-1]:H & E- Intramembranous formation of spongy 
bone. Note the periosteum adjacent to the bony 
spicules. Osteocytes can be seen within lacunae 
surrounded by bony matrix.

[5-2]:H & E- Osteoblasts line the surface of the 
spicules depositing osteoid which appears as a 
lighter staining area immediately beneath the 
osteoblasts.

[5-3]:H & E- Osteoclasts are large multinucleated 
cells that can reabsorb and remodel bone.

[5-4]:The next three slides demonstrate the 
conversion of spongy bone to compact bone. Initially 
spicules of spongy bone fuse and entrap mesenchymal 
connective tissue containing a blood vessel.

[5-5]:H & E- The developing bone entraps osteocytes 
within a forming Haversian system. Lamellae are not 
yet distinguishable.

[5-6]:H & E- Eventually distinct lamellae encircling 
the central Haversian canal can be observed. The 
entrapped osteocytes exhibit filopodia within the 
canaliculi.

[5-7]:A ground section of compact bone exhibiting the 
central canal of a Haversian system (pointer). 
Between the lamellae occurs lacunae wherein 
osteocytes normally reside.

[5-8]:TEM 4-1. 
An odontoclast (osteoclast of teeth). Note this 
multinucleated (1) cell is surrounded by dentin (2),  
and dentinal tubules (3). The odontoclast cytoplasm 
exhibits a ruffled border (4), vacuoles (5) and 
mitochondria (6).

[5-9:SEM 4-4. A forming osteocytic lacuna shows its 
inside surface pockmarked with openings of 
canalicular channels (1). Newly deposited mineral 
matrix is seen roofing over and around (2) the 
lacuna.

[5-10]:SEM 4-2.  A low power scanning electron 
micrograph of spongy bone showing ongoing bone 
deposition (1) on the right side, and ongoing 
osteoclastic bone resorption (2) indicated by eroded 
(scalloped) surface on the left half of the electron 
micrograph. Marrow space (3) and forming osteocytic 
lacunae (4).  

[5-11]: Development of long bones occurs through 
formation of a primary ossification center within the 
diaphysis. At later stages in embryonic development a 
secondary ossification center arises by replacement 
of the hyaline cartilage with an epiphysis.

[5-12]:H & E- Neonates up to the beginning of 
adulthood exhibit elongation of long bones with a 
distinct diaphysis, epiphyseal growth plate (hyaline 
cartilage) and epiphysis.

[5-13]:The epiphyseal growth plate of hyaline 
cartilage is partially reabsorbed and ossified 
leading to endochondral bone development.

[5-14]:During endochondral bone development at the 
trailing edge of the epiphyseal growth plate the 
hypertrophied chondrocytes progressively degenerate 
allowing the reabsorption of cartilage and the 
deposition of osteoid by osteoblasts. 

[5-15]:Behind the epiphyseal growth plate 
intermingled with the spicules of the spongy bone can 
be observed the bone marrow (lines).